1. Field of the Invention
The present invention generally relates to a chemical mechanical polishing (CMP) process, and more particularly, relates to a method for reducing dishing effects during a chemical mechanical polishing process.
2. Description of the Prior Art
As integrated circuit device dimensions have decreased, it has become increasingly common within advanced integrated circuits to employ trench isolation methods, such as shallow trench isolation (STI) methods, to form trench isolation regions nominally coplanar with adjoining active semiconductor regions of semiconductor substrates. Such STI methods typically employ a chemical mechanical polishing (CMP) method to provide a nominally planarized surface to a trench fill dielectric layer formed within the trench. Trench isolation regions nominally coplanar with active semiconductor regions within semiconductor substrates are desirable since they optimize, when subsequently forming patterned layers upon those nominally coplanar trench isolation regions and active semiconductor regions, the limited depth-of-focus (DOF) typically achievable with advanced exposure tooling.
CMP is a process used to create smooth, planar layers on wafers, which essentially provides for polishing a wafer by rubbing a polishing pad against the wafer to grind the surface layer. The polishing pad is saturated with an abrasive slurry solution that may aid the planarization. A common slurry used for polishing silicate film is colloidal silica in an aqueous KOH solution. CMP tools are well known in the art. Such tools are disclosed in U.S. Pat. Nos. 3,841,031 and 4,193,226. The tools include a polishing wheel with the wafer attached to the wheel. As the wheel rotates, the wafer is forced against a wetted polishing surface and the surface is planarized.
During a CMP process applied on a silicon dioxide layer, there is a thin hydrated silica gel layer created over the surface of the silicon dioxide layer. The reaction may be described by the following formulas:
Sixe2x80x94Oxe2x80x94Si+H2Oxe2x86x922SiOH 
(SiO2)x+2H2Oxe2x86x92(SiO2)xxe2x88x921+Si(OH)4 
According to the above equations, a global planarization is achieved by rapidly and repeatedly removing the readily formed hydrated silica gel layers. In an aqueous environment, the surface undergoes a hydration reaction with H2O to produce a surface network of hydroxylated Si molecules. Dissolution of this network generally occurs above a pH of 9.0 because of the solubility of the reaction product at high pH. Typically, the formation of Si(OH)4 is accelerated in a more alkaline environment. A most common composition of slurry for polishing silicate films includes abrasive, for example, fumed silica, deionized water, alkaline solvent, and buffer solution. A normal pH for this composition is usually between 10.5 and 11.5.
While trench isolation methods employing CMP planarizing of trench fill dielectric layers formed into isolation trenches are therefore quite desirable and common in the art of advanced integrated circuit fabrication, such trench isolation methods are nevertheless not entirely without problems. In particular, it is known in the art that trench isolation methods employing CMP planarizing of trench fill dielectric layers formed into isolation trenches often, when the isolation trenches are comparatively wide, provide isolation trenches where a planarized trench fill dielectric layer formed therein is dished.
In order to totally remove the trench fill dielectric layer above a silicon nitride stop layer, the silicon nitride stop layer is strategically over-polished. Since the hardness of silicon nitride is higher than that of the trench fill dielectric layer, the trench fill dielectric layer is polished away with a higher rate to cause a dishing top surface of STI. The dishing phenomenon affects the performance of the device.
Since dishing within planarized trench fill layers formed within trenches within integrated circuits is generally undesirable, it is therefore in general towards forming, without dishing, planarized trench fill layers within trenches within integrated circuits that the present invention is directed. Methods for preventing dishing during a CMP process are known in the art of integrated circuit fabrication. For example, Bose et at., in U.S. Pat. No. 5,492,858 discloses a method for forming a planarized trench fill dielectric layer within an isolation trench within an integrated circuit. The method employs, within the isolation trench, a barrier layer that allows for densifying, through steam annealing, a conformal silicon oxide trench fill dielectric layer, which may then subsequently be planarized, while avoiding dishing, through a chemical mechanical polish (CMP) planarizing method.
In addition, Homma et at., in U.S. Pat. No. 5,420,075 discloses a method for selectively depositing insulator layers into inter-metal spacings within patterned metal layers within integrated circuits. The method provides for treating a portion of a patterned metal layer with a fluorine containing reactive ion etch (RIE) plasma to inhibit formation of a chemical vapor deposited (CVD) silicon oxide layer upon that portion of the patterned metal layer.
It is an object of this invention to provide an effective and simple method for reducing dishing of a polished silicate film during a CMP process.
It is a further object of this invention to provide a method for reducing dishing during an isolating trench formation process by using a polishing slurry to which is added a quantity of alcohol and/or phenol before the polishing end point.
According to one aspect of the present invention, a method for reducing dishing effects is provided. The method is applied to polish a surface of a wafer containing a silicate film thereon. The method comprises using a polishing slurry containing an organic alkyl or aryl compound with at least one hydroxyl group (i.e. ROH compound) during the processing of polishing the silicate film. An organic hydrophobic layer is created over the silicate film in contact with the ROH compound, thus alleviating the undesirable dishing effects. The organic hydrophobic layer is thereafter cleaned using ozone-containing deionized water.
According to a further aspect of the present invention, the present invention comprises the following essential chemical mechanical polishing steps:
(1) starting a chemical mechanical polishing process, at a polishing time=0 second, by using an alkaline slurry containing water, abrasives and soluble inorganic salts;
(2) at a polishing time=t1 second, adding a predetermined quantity of alcohol and/or phenol into the alkaline slurry;
(3) at a polishing time=t2 second, detecting an end point of the CMP process, wherein t2 greater than t1; and
(4) terminating the CMP process.
To promote the formation of the hydrophobic layer, the pH of the slurry is controlled to between 7 and 13. It is advantageous that by addition of the ROH compound into the slurry during the CMP process, a hydrophobic layer is created over a silicate film, thereby reducing dishing of the silicate film in a wide trench. In addition, the organic hydrophobic layer is easily removed by ozone-containing water.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.